Light-emitting elements utilizing electroluminescence (EL) are highly anticipated as next-generation display technology. In recent years, research and development has been extensively conducted on such light-emitting elements. In a basic structure of the light-emitting element, a layer containing a light-emitting substance is provided between a pair of electrodes. By applying voltage to the element, light emission can be obtained from the light-emitting substance in an excited state.
The light-emitting element is a self-luminous element and thus has advantages over a liquid crystal display element, such as high visibility of the pixels and no need for backlight, and is considered to be suitable as a flat panel display element. Another major advantage of the light-emitting element is that it can be fabricated to be thin and lightweight. Besides, very high response speed is also a feature of the light-emitting element.
Furthermore, the light-emitting element can be formed in a film form, and thus makes it possible to provide planar light emission easily. Thus, a large-area element utilizing planar light emission can be formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Thus, the light-emitting element also has great potential as a planar light source that can be used for a lighting device and the like.
Light-emitting elements are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by applying voltage to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer including the light-emitting organic compound, whereby current flows. Then electrons and holes (i.e., carriers) are recombined, so that the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. Note that in fabrication of a light-emitting element, characteristics of the light-emitting element are greatly affected by such light-emitting substances.
A method has been disclosed in which a novel fluorescent material with high emission efficiency is used for a light-emitting layer included in a light-emitting element to provide a highly efficient light-emitting element (see, for example, Patent Document 1).